Synthesis and biological studies of 5-aminolevulinic acid-containing dendrimers for photodynamic therapy.

Using a convergent growth approach, a series of novel 5-aminolevulinic acid (ALA)-containing dendrimers have been synthesized. In these molecules, ALA residues are attached to the periphery by ester linkages, with amide bonds connecting the dendrons. Three first-generation dendrimers, bearing either 6 or 9 ALA residues, were synthesized by attachment of a tris(Boc-protected ALA)-containing wedge (1) to a di- or tripodent aromatic, or tripodent aliphatic core. Two second generation 18-ALA-containing dendrimers were also synthesized using a 3,3'-iminodipropionic acid spacer unit between wedge 1 and the aromatic core. These compounds differed only in the distance between the core and the linker unit. The Boc-protected dendrimers were deprotected using trifluoroacetic acid and isolated as their TFA salts. The potential of these ALA ester dendrimers as macromolecular prodrugs for photodynamic therapy has been demonstrated in the tumorigenic keratinocyte PAM 212 cell line.

Photosensitization of pancreatic tumour cells by delta-aminolaevulinic acid esters.

A series of straight chain, branched and cyclo-delta-aminolaevulinic acid (ALA) esters have been synthesized and their photosensitizing properties analysed using an in vitro system of rat pancreatoma cells. Structurally favourable ALA esters not only induced the formation of more of the endogenous photosensitizer, protoporphyrin IX (PpIX), but they did so at a faster rate than ALA itself. This action was reflected in a substantial increase in photocytotoxicity of some 270 times, using the more potent ALA esters. An important structural feature was identified in two of the ALA esters which greatly limited PpIX production, i.e. a branch point located next to the site of ester cleavage. Experiments on the transport of ALA and of ALA esters across the cell membrane showed that ALA, but not ALA esters, gain access to the cell via the di- and tripeptide transporter, PEPTI. Finally, these results show that the esterification of ALA can greatly increase its cellular uptake, so generating more intracellular PpIX, improved tumour cell photosensitization and enhanced photocytotoxicity.